https://www.youtube.com/watch?v=3m5qxZm_JqM
"What do you do to protect the environment in this case?"
"The planes fly outside the environment."
"Into another environment?"
"No, no, they fly beyond the environment. They're not in an environment."
"But it must be somewhere. What's out there?"
"Nothing's out there."
"There must be something out there."
"There's nothing out there. All there is is air, and clouds, and birds."
"And?"
"Twenty thousand tons of tetraethyllead."
"What else?"
"CO2 emissions, CFCs, and about seven hundred 737 Max's. The environment's perfectly safe."
https://en.wikipedia.org/wiki/Avgas#Phase-out_of_leaded_avia...
> 70% of 100LL aviation fuel is used by the 30% of the aircraft in the general aviation fleet that cannot use any of the existing alternatives
On the second point: lead is specifically added to help increase the lifetime of components like spark plugs, since it decreases knocking.
"Knocking" is an antiquated term. Knocking = detonation, and any severe detonation would destroy an aircraft engine. For example, this is why we take off with our mixture so rich, to create a huge margin against detonation by lowering internal cylinder pressures.
A large part of this problem, AFAIU, is that "cannot use" can mean lack of type certification. IOW, as a practical matter many (most?) pre-existing engines could use new, unleaded fuels, but engine certifications haven't yet been updated by the FAA. Only leaded-fuel is legally permitted in most old engine types, and old engines dominate in general aviation.
See, for example, this blurb further down in that Wikipedia article,
> In 2022, Paul Bertorelli of AVweb reported that the FAA is dragging its feet on broadly certifying G100UL, delaying approval of the fuel for more engines and spending over $80 million on EAGLE to re-start a search for an unleaded fuel when G100UL has been under evaluation for over 10 years.
where G100UL is (I think) the only alternative replacement fuel actually approved so far, certified just last year, and only for one particular family of engines.
No one serious would ask what was wrong with tetraethyllead in gasoline fueling automobiles in urban areas, but aviation is a completely different context.